Stripping metal coatings



2,872,361 srmrrmo METAL COATINGS Howard T. Siefen, Jackson, Mich., and John M. Campbell, Burlington, Iowa, assignors to the United States of America as represented by the United States Atomic Energy Commission No Drawing. .Application April 2, 1946 Serial No. 659,134 I .SClaims. (Cl.'134- 7) This invention relates to the removal of metallic coatings from base metals and isparticu larly concerned with improvements in the removal of aluminum-silicon alloy coatings from metallic uranium.

In order to protect metallic uranium in certain uses of this material, for example in the manufacture of plutonium, it is necessary to temporarily protect the uranium from corrosive influences by means of a protective metallic coating. Aluminum-silicon alloy coatings or aluminum jackets or sheaths bonded to the metallic uranium by means of aluminum-silicon alloy bonding solders have been employed for this purpose. For salvage and recovery purposes, it is desirable to remove the protective metal coating from the metallic uranium.

An object of the present invention is the provision of a method suitable for removing aluminum-silicon alloy coatings bonded to metallic uranium. A further object is to remove the aluminum-uranium-silicon alloy layer normally formed at the surface of the metallic uranium during application of aluminum-silicon alloy coatings. A still further object of the invention is the removal of protective coatings comprising an aluminum layer bonded to the metallic uranium by means of aluminum-silicon alloy bonding metal. A further object is the accomplishment of these purposes with a minimum of chemical reagents. A further object of the invention is the removal of aluminum-uranium-silicon alloy from metallic uranium without substantial loss of the base metal.

Further objects will be apparent from the following general description and detailed discussion of the invention.

In accordance with the present invention aluminumuranium-silicon alloy bonded to metallic uranium base metal is subjected to treatment with hot concentrated nitric acid to partially dissolve and embrittle the alloy and the embrittled alloy is then shot-blasted to loose it I from the uranium. It has been found that by treatment of the aluminum-uranium-silicon alloy with hot nitric acid the alloy is not only dissolved butis embrittled to such an extent that residual coating metal is readily amenable to removal by shot-blasting. This mechanical removal of a part of the coating permits use of less chemical reagent than would be required for complete solution of the coatings. The nitric acid treatment and the shot-blasting treatment may be conducted alternately and repeatedly to secure the desired results. A maximum saving of reagent is secured in this manner. Normally, however, a sufiicient saving is not justified to warrant more than two or three repetitions of the treatments.

In application of the process to metallic uranium bodies protected by a metallic aluminum can or sheath bonded to the metallic uranium by means of aluminum-silicon bonding alloy, a preliminary treatment may be (but need not be) employed for removing the aluminum sheath and an additional treatment for removing the aluminumsilicon alloy down to the aluminum-uranium-silicon layer. An efiective method for removing the metallic aluminum comprises a treatment with hot aqueous 10% HNO United States Pate .0 7

i 7 however, to use srnoothtspheric a l 2,872,361 Patented Feb. 3, 1959- solution containing about 0.1% mercury in theformwof mercuric nitrate. The metallic aluminum is rapidly dissolved by this reagent. The aluminum-silicon alloy is readily removed by treating it with aqueous 20% tel 30% caustic soda solution at a temperaturev of. to C. When all of the metal readily, removableby these treatments has been removed from the base metal,

ings of the present invention. l I For chemical treatment of the aluminum-uraniumesilicon coating aqueous 50%. nitric acid solution employed; for periods between Sand 30 minutes at temperatures of, 60 to 70 C. may be used to advantage. The shot-blasting treatment may be carried out rio Sizes d ha 9f @P rt da- .i; Pre e re t ,1... stslistzhar psesdis afneter in the neighborhood of 0.05 inch. By use of such particles abrasion is reduced to a minimum while removal by fracture of brittle metal is accomplished in a very eifective manner.

The following examples further illustrate the invention. When not otherwise indicated parts are expressed in terms of weight.

it is ready for the treatment in accordance withthe teach Example 1 The metallic uranium comprised rods originally having a diameter of 1.358 inches and a length of about 8 inches coated with metallic aluminum sheaths bonded to the metallic uranium by means of aluminum-silicon alloy. These rods were immersed in nitric acid of 10% concentration containing 0.1% mercury in the form of mercuric nitrate to remove the aluminum sheath. In this operation the concentration of nitric acid was maintained by addition of 70% HNO as the nitric acid became consumed. When the aluminum had been entirely stripped from the aluminum-silicon bonding alloy, the rods were immersed in 20% NaOH solution at a temperature of about 110 C. to remove the aluminum-silicon alloy. Between 220 and 230 grams of NaOH removed about 60 grams of aluminum-silicon alloy from each rod. Each rod was given a short shot-blasting with 0.048-inch steel shot to remove loose material and was then treated with 50% HNO solution at about 65 C. for 5 minutes. This treatment removed all of the aluminum-uraniumsilicon alloy from portions of the base metal and left portions of the base metal covered with a darkened coating of the alloy in an embrittled form. The rods were then shot-blasted with 60-mesh shot (0.048-inch diameter) at about 65#/sq. in. air pressure for 20 minutes. Essentially all of the embrittled compound layer was removed by this treatment. The final diameter of the rods was 1.357 inches.

Example 2 Metallic uranium rods, from which an aluminum-silicon bonded aluminum sheath had been removed with mercury catalyzed nitric acid and caustic soda treatments as described in Example 1, were treated for 8 minutes in aqueous 50% nitric acid at 60 to 70 C., then shotblasted for 25 minutes with 30 mesh steel shot, at about 65#/sq. in. air pressure then treated again for five minutes in 50% nitric acid at 60 to 70 C. Substantially complete removal of the aluminum-uranium-silicon alloy layer was obtained without material removal of base metal.

It will be understood that we intend to include variations and modifications of the invention and that the preceding examples are illustrative only and in no wise to be construed as limitations upon the invention, the scope of which is defined in the appended claims, wherein we claim:

1. The method of removing aluminum-uranium-silicon alloy bonded to metallic uranium, which comprises subjecting the aluminum-uranium-silicon alloy to treatment with hot nitric acid to partially dissolve and embrittle the alloy, and shot-blasting the embrittled alloy to loose it from the uranium.

2; The method of removing aluminum-uranium-silicon alloy bonded to metallic uranium, which comprises subjecting said alloy to a repetition of alternate treatments of hot nitric acid and shot-blasting.

3. The method of removing aluminum-silicon alloy bonded to a metallic uranium article with an interface of aluminum-uranium-silicon alloy which comprises dissolving the aluminum-silicon alloy with hot aqueous caustic alkali solution to expose said interface of aluminumuraniurri-silicon alloy, subjecting the interface alloy to treatment with hot concentrated nitric acid to partially dissolve it-and embrittle undissolved portions, and shotblasting the article to loose embrittled alloy from the uranium.

4. The method of removing aluminum-silicon alloy bonded to a metallic uranium article with an interface of aluminum-uranium-silicon alloy Which comprises dissolving the aluminum-silicon alloy with hot aqueous caustic alkali solution to expose said interface of aluminumuranium-silicon alloy, subjecting the interface alloy to treatment with hot concentrated nitric acid to partially dissolve it and embrittle undissolved portions, shot-blasting thearticle to loose embrittled alloy from the uranium, and treating the shot-blasted article with hot concentrated nitric acid to remove residual aluminum-uraniumsilicon alloy.

5, The method of removing aluminum-silicon alloy bonded to a metallic uranium article with an interface of aluminum-uranium-silicon alloy which comprises dissolving the aluminum-silicon alloy with hot aqueous caustic alkali solution to expose said interface of aluminumuranium-silicon alloy, and then subjecting the interface alloy to a repetition of alternate treatments of hot nitric acid and shot-blasting.

No references cited. 

3. THE METHOD OF REMOVING ALUMINUM-SILICON ALLOY BONDED TO A METALLIC URANIUM ARTICLE WITH AN INTERFACE OF ALUMINUM-URANIUM-SILICON ALLOY WHICH COMPRISES DISSOLVING THE ALUMINUM-SILICON ALLOY WITH HOT AQUEOUS CAUSTIC ALKALI SOLUTION TO EXPOSE SAID INTERFACE OF ALUMINUMURANIUM-SILICON ALLOY, SUBJECTING THE INTERFACE ALLOY TO TREATMENT WITH HOT CONCENTRATED NITRIC ACID TO PARTIALLY DISSOLVE IT AND EMBRITTLE UNDISSOLVED PORTIONS, AND SHOTBLASTING THE ARTICLE TO LOOSE EMBRITTLED ALLOY FROM THE URANIUM. 